This invention relates to a redundancy arrangement for a local area network, and particularly to a redundancy arrangement for a multiple access communication system having a bidirectional communication bus.
Local area networks (LAN'S) have been developed for interconnecting various data using devices, such as general purpose computers, personal computers, remote terminals, and peripheral devices, which are situated in limited geographical areas, such as an office building or a research center. A basic characteristic of local area networks is that all using devices are continuously and physically attached to a common communication medium, such as a coaxial cable, whereby each using device may selectively communicate with any other using device on the network. Typical communications provided by a LAN in the modern business environment include those necessary for office automation, management information systems, management graphics, production control, word processing and voice and video applications. Because of the large volume of communications which a LAN may be carrying, reliability of the LAN, and in particular the communication medium, is of great importance.
The most common communication medium for local area networking is coaxial cable; however, other suitable medium such as fiber optic cable have also been used.
Due to various losses in the communication medium, the implementation of some classes of LANs over the geographical area represented by an office complex or a research facility generally requires the use of several cable segments tied together across repeater stations which receive, amplify and retransmit the communication traffic. If any one of the cable segments is damaged, all or part of the network could be rendered unusable.
For example, a typical application of a baseband LAN installed in a high rise building involves the use of at least three segments of cable. Separate horizontal cable segments installed on each of at least two floors of the building are interconnected by a single cable segment, referred to as a vertical riser, which is tied to each horizontal cable segment via respective repeater stations. Data using devices connected to a horizontal cable on one floor may then communicate with using devices on another floor of the building via the vertical riser. If the vertical riser cable should be damaged, this could prevent a using device on one floor of the building from communicating with a using device located on another floor.
It would thus be desirable to have a redundancy arrangement to avoid this risk of failure in the communication medium.
In a LAN of the type comprising a multiple access communication system having a bidirectional communication bus, such as the now well known ETHERNET LAN, the provision of a redundancy arrangement for the communication medium is not a straightforward problem. In such a system there can be only one path between any two using devices connected to the communication medium in order to prevent data transmitted from one using device from colliding with itself somewhere on the medium and thus resulting in a jumbled transmission. Consequently, in providing redundancy in the communication medium for a multiple access system having a bidirectional communication bus, care must be taken to avoid simultaneous alternative paths for the transmission of data between any two using devices connected to the system.